Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element). An inorganic electroluminescent element has been utilized as a flat light source, however, requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emission layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emission layer to be recombined, resulting emission utilizing light release (fluorescence•phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
In an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption. Examples of such technologies are disclosed in JP 309379 in which a slight amount of a fluorescent substance doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element. Further, in JP-A 63-264692, there are disclosed such as an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance. In JP-A 3-255190, an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye is disclosed.
Regarding to the technologies disclosed in the above-described patent documents, when emission from an excited singlet is utilized, since a generation ratio of a singlet exciton to a triplet exciton is 1:3, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of a quantum efficiency (next) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from researchers of Princeton University (M. A. Baldo et al., Nature, Vol. 395, pages 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active (refer to M. A. Baldo et al., Nature, Vol. 403, No. 17, pages 750-753 (2000) and U.S. Pat. No. 6,097,147).
Further, in the organic EL element utilizing phosphorescent emission which was discovered recently, it is possible to realize a light emission efficiency of about four times larger in principle than an element utilizing conventional fluorescent emission. Therefore, studies and developments of a layer structure and an electrode of a light emission element, as well as developments of materials for the above organic EL element, have been carried out. For example, in a paper of “S. Lamansky et al., J. Am. Chem. Soc., Vol. 123, p. 4304 (2001)”, studies on synthesis of many compounds by focusing on heavy metal complexes such as iridium complexes are described.
Since the organic EL element is an all solid state element composed of a film of an organic material exhibiting about 0.1 μm in thickness in between two electrodes, and the light emission can be achieved with a relatively low voltage of 2 to 20 volts, the organic EL element is a promising technology for use as the next-generation flat display or lighting device.
However, since the mechanism of light emission of the organic EL element is based on a light emission utilizing a deactivation from the excited state to the ground state of organic materials, a large band gap is required to emit light in the region of short wavelength such as blue or green. Therefore, a high voltage is required to cause an excitation having the above large band gap.
Further, since the excited state itself exists at a high level, the damage is large when returning to the ground state, and then an emission life tends to be shortened compared to green or red emission, and in particular, the phosphorescent emission utilizing light emission from an excited triplet significantly exhibits the above tendency of the shortened life.
There have been various technologies to dissolve the above-described problems. Examples include a technology where a constituting layer of an organic electroluminescent element is formed, and then the layer is polymerized, and in which technology bifunctional triphenylamine derivatives having two vinyl groups in their molecules are disclosed, and after the film formation of the compound, a three-dimensionally cross-linked polymer is formed (for example, refer to Patent Document 1); a technology where materials having two or more vinyl groups are incorporated into a plurality of layers, where the polymerization reaction is carried out via irradiation of the ultraviolet rays or heat at a step of forming an organic layer before an cathode is laminated (for example, refer to Patent Document 2); a production technology where a polymerization reaction is allowed to proceed during a film formation by adding an AIBN (azoisobutyronitrile), a radical generating agent, into a mixture of a material having vinyl groups at terminals of a phosphorescent light-emitting dopant and a comonomer similarly having vinyl groups (for example, refer to Patent Document 3); and a production technology where a Diels-Alder reaction is allowed to occur between two molecules within the same layer to result in a linkage (for example, refer to Patent Document 4).
As described above, the phosphorescent light-emitting dopant, in particular a phosphorescent light-emitting dopant applicable to blue, exhibits a large band gap. Therefore, materials which may be usable as a host for such dopant, achieve a high light emission efficiency and a long life at the same time, and further may be applicable to a coating method (also referred to as a wet process) have not been known.
Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 5-271166
Patent Document 2: JP-A No. 2001-297882
Patent Document 3: JP-A No. 2003-73666
Patent Document 4: JP-A No. 2003-86371